The present invention relates to a gas turbine combustor and, more particularly, is concerned with a low NOx gas turbine combustor provided with a main-fuel line of a pre-mixing lean-burn system and a sub-fuel line of a diffusion combustion system.
In general, a main factor for the generation of NOx in a gas turbine combustor resides in that a combustion area in which an equivalent ratio of fuel and air is nearly "1" is formed in a combustion gas and a temperature of the combustion gas in this combustion area is locally highly raised.
The NOx thus generated due to such factor is suppressed in conventional art by mixing a supply fuel with an air of an amount more than that necessary for the combustion to dilute the mixture or by supplying, to the combustion area, the mixture in which the fuel is preliminarily uniformly mixed with the air.
Concerning the pre-mixing lean-burn system, a combustor system has been generally utilized which is provided with a main fuel line of the pre-mixing lean-burn system and a sub-fuel line of the diffusion combustion system, in consideration of covering a wide operation range. This is based on the fact that the pre-mixing lean burn system is superior for the low NOx burning, but another diffusion combustion system is required in order to keep a combustion flame in a wide operation range.
FIG. 6, mentioned hereinlater, shows one example of a conventional gas turbine combustor, in which a downstream end of a fuel supply base line 2 for supplying a fuel is branched, for a combustion liner 3, into a main fuel line 4 for the pre-mixing lean-burning and a sub-fuel line 5 for the diffusion combustion. The generation of the NOx largely depends on the fuel supply ratio in the diffusion combustion line 5, so that, in order to reduce the generation of the NOx, it is desired to possibly minimize the combustion in the diffusion combustion line 5.
Usually, in the gas turbine combustor, an air-fuel ratio is made small from an ignition time to an intermediate load operation time for a gas turbine, a temperature of the flame is hence low, and the NOx is less generated, so that the pre-mixing lean-burning line as the main fuel line 4 is not utilized and the operation control of the gas turbine can be mainly made through the diffusion combustion line as the sub-fuel line 5.
However, in a load operation period of the gas turbine after the switching to the intermediate load operation mode, distribution of the fuel supply to the main fuel line 4 and the sub-fuel line 5 is regulated by locating a fuel flow rate control valve 6 and fuel distributing valves 7 and 8 for the main and sub-fuel lines 4 and 5, respectively, and controlling degrees of openings of these valves 7 and 8 by a fuel supply control unit 9 in consideration of requirement for a gas turbine operation start mode and a load operation mode.
In the gas turbine combustor of the structure described above, however, the distribution of the fuel into the main fuel line and the sub-fuel line with respect to the respective operation modes is controlled as shown in FIG. 7, mentioned hereinlater. Accordingly, it becomes important to suitably design main and sub-fuel nozzles 10 and 11 so as to conform with the fuel flow rates, and namely, it is necessary to suitably set fuel nozzle areas. The fuel flow rates passing the main and sub-fuel nozzles 10 and 11 are decided by fuel rates at fuel inlet ports, a pressure difference between pressures before and after the passing of the main and sub-fuel nozzles 10 and 11, and the fuel nozzle areas.
A fuel supply pressure necessary for the flow rate of the supply fuel with respect to the fuel nozzle area changes as shown in FIG. 8, mentioned hereinlater, but the sub-fuel line generates a peak pressure against the rapid change of the required fuel at a point before and after the switching load described above. As will be understood from FIG. 8, in the load range of 0 to 100 %, the maximum fuel supply pressure is not decided on the main fuel nozzle at the 100 % load time, but decided by the sub-fuel nozzle at a load point before and after the above switching load. This is based on the fact that, generally, with respect to the setting of the fuel nozzle area, the nozzle pressure ratio, i.e. (fuel supply inlet pressure)/(nozzle outlet pressure), at the fuel nozzle portion will cause instable phenomenon such as combustion oscillations when the ratio becomes below a certain limit value, and for this reason, the nozzle areas of the fuel nozzles of the main and sub-fuel lines 4 and 5 so that the nozzle pressure becomes higher than the limit nozzle pressure ratio in all the operation range.
Particularly, with respect to the sub-fuel line 5, the fuel nozzle area is set so that the nozzle pressure ratio becomes larger than the limit nozzle pressure ratio in the operation range at an operation load of more than the switching load at which the fuel nozzle ratio is likely made small. On the contrary, in the operation range below the switching load, it is necessary to solely flow the fuel likely as the conventional gas turbine combustor. Accordingly, with respect to the small fuel nozzle area, the supply gas pressure is to be made considerably high in comparison with the conventional diffusion combustion type gas turbine combustor as shown in FIG. 8, thus being troublesome.
As described hereinbefore, since the amount of the NOx generated in the combustor depends mainly on the location of the diffusion combustor in the sub-fuel line 5, in order to reduce the generation of the NOx during the operation mode more than the switching load, it will be necessary to possibly reduce the distribution of the fuel to the sub-fuel line 5. Accordingly, in this meaning, the fuel supply pressure peak becomes more remarkable as the reduction of the NOx is strongly intended. Furthermore, in a large-sized power plant, the supply gas fuel is supplied by increasing a pressure of the low liquid state fuel to a working pressure by means of a pump and then supplying the same in a gas state, but in an intermediate or small sized power plant or in a city use power plant, a gas of a low pressure of about 0.5 to 1.5 kg/cm.sup.2 is supplied to the gas turbine combustor by increasing its pressure to a pressure necessary for the gas turbine combustor. Accordingly, when the supply gas fuel pressure increases as in the conventional example described above, not only the working power of the gas fuel compressor increases but also the design of the gas fuel compressor becomes itself difficult, and a pressure withstanding capability of the associated equipments or machinaries must be made increased, resulting in adverse plant working efficiency, cost-up and problem of stable operation.